Ovalicin (1) is a metabolite of Pseudorotium ovalis possessing antibiotic, antitumor, immunosuppressve, and seed germination stimulating properties. Fomannosin (2) is a phytotoxic metabolite of the economically important wood-rot fungus Fomes annosus. Pentalenolactone (3) is a Streptomyces metabolite with a broad spectrum of antimicrobial activities, being particularly effective against phytopatogenic fungi. The structure of each of hese sesquiterpenes presents a unique and complex biosynthetic problem, not readily amenable to traditional radioisotopic labeling and degradation techniques. We are examining the biosynthetic problems posed by these substances. The goals are to: 1) elucidate the rearrangement pathway by which ovalicin is formed, and to identify the key intermediates along this pathway; 2) to provide mechanistic and stereochemical details bearing on a proposed scheme for the biosynthesis of fomannosin; 3) to establish a biosynthetic scheme for the formation of pentalenolactone; 4) to examine the application of powerful instrumental techniques, carbon and deuterium magnetic resonance, to biosynthetic problems; and 5) to probe and extend the utility of homo-and heteronuclear double labeling in magnetic resonance studies of isoprenoid biosynthesis.